So the RCW for the tax says that it's based on the MSRP of the car on a depreciation schedule. However, it refers to the definition of MSRP as defined in a another RCW regarding trailers. And in that definition, it very clearly says that it's based on the base model, not counting for any upgrades.
Tesla is selling this car as a "Model 3". The "Model 3" base price is $35,000. It is an option (even though it's mandatory right now) to get the $9k LR. It's also an option to get the Premium Interior at $5k. And it's also an option to change the paint color, or the wheels. So, you are supposed to be taxed on $35,000 on a depreciation schedule. In my MVPA, it clearly shows $35k for the Model 3, and then it says Long Range Dual Motor is $18k (because it was when AWD was $4k, and I assume it is adding $5k for the PUP). And the RTA tax they gave me is indeed based on $35k.
Go to
https://fortress.wa.gov/dol/extdriveses/NoLogon/_/, and in the lower-right corner, click "Start" and walk through it. Now, here's the tricky part. When you walk through, if you tell it you don't have a VIN and it prompts you for information... If you choose 2017 for the car year, and choose Tesla Model 3, it will give you two sub-choices: "Standard" with a price of $35k, and "Long Range" with a price of $44k. For 2018, though, it only shows "4dr Standard", which is says is a RWD.
So they may still be working through the kinks. It seems based on the reading of the law that $35k is correct, because Tesla sells a "Model 3" and everything else are options...